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The chemical properties of an element depend on the electronic configuration of the outershell
...

According to the ground state electronic configuration of carbon, carbon is divalent
...
Some energy must be supplied to the
system in order to effect this promotion
...

6C

1s2

2s2

2px

1s2

2p y 2p z

Carbon in ground state

2s1

2px

2pz

2py

Carbon in excited state

The four valencies of carbon atom are similar and they are symmetrically
arranged around the carbon atom
...
They are directed towards the
1
...

Bonding in organic compounds
...
Carbon must
form only covalent bonds, i
...
, it should share its valency electrons with other atoms
...
The overlapping is possible
by two ways,
(1) End to end overlapping : This type of overlapping is possible between s

s, s

p x and p x

px

atomic orbitals
...

(2) Sidewise or parallel or lateral overlapping : Such overlapping is possible between p
orbitals
...

-Bond
Formed by End to End overlap of AO’s
...


p atomic

Has cylindrical charge symmetry about bond

Has maximum charge density in the cross-

axis
...


Has free rotation

No free rotation, i
...
, frozen rotation

Low energy

Higher energy

Only one

One or two

bond can exist between two atoms,

bonds can exist between two

atoms
...
Thus the geometry
of the molecule depends on the

bonds
...
Geometry of the
molecule not depends on

bond
...


weaker
...


bond always exist along with a

bond and

bond is formed after the formation of
bond
...

(1) The process of mixing atomic orbitals to form a set of new equivalent orbitals is termed as
hybridisation
...
These are,
(i) sp 3 hybridisation (involved in saturated organic compounds containing only single covalent bonds),
(ii) sp 2 hybridisation (involved in organic compounds having carbon linked by double bonds) and
(iii) sp hybridisation (involved in organic compounds having carbon linked by a triple bonds)
...
5

Tetrahedral

25 or 1/4

sp2

1s and 2p

One

Three -

120

Trigonal

33
...

Number of – bond/s

0

1

2

Type of hybridisation

sp3

sp2

sp

Examples : (i) CH 3

O
||
C CH 3

sp 3

sp 2

CH 3

CH

CH

CH 2

sp 2

sp 3

sp

CH 2

sp

sp 2

N

(iv) HC

C

CH

CH 2

sp

sp

sp

sp 2

sp 2

C

sp 2

C

sp 2 sp 2 sp 3

sp 3

(iii)

CH

(ii) CH 2
sp 2

CH

Note :  In diamond carbon is sp3 hybridised and in graphite carbon is sp 2 hybridised
...

(ii) Second method : (Electron pair method)
ep = bp + lp; where ep = electron pair present in hybrid orbitals , bp = bond pair present in hybrid orbitals
Number of bp = Number of atoms attached to the central atom of the species
Central atom

Central atom

First atom

H

1

H

2

H C

C H

C=C
H

Second atom

bp

H
bp = 3

1

2

H

2
3

bp = 3

Third atom

Number of lp’s can be determined as follows,
(a) If carbon has

- bond/s or positive charge or odd electron, than lp on carbon will be zero
...

Number of electron pairs (ep) tells us the type of hybridisation as follows,
ep

2

3

4

5

6

Type of hybridisation

sp

sp 2

sp 3

sp 3 d

sp 3 d 2

Example :

(i) CH 2

CH

(ii) CH 2

bp 2
lp 0
ep 2, sp

bp
lp
ep

CH (iii) CH 2
2
1

C
|

(iv) CH

CH 3

ep

CH 3

bp 1
lp 1
ep 2, sp

CH 3
bp 3
lp 0

3, sp 2

C (v)

CH CH 3

bp 3
lp 1
ep

4 , sp 3

3, sp 2

(3) Applications of hybridisation
(i)

Size of the hybrid orbitals : Since s - orbitals are closer to nucleus than p - orbitals, it is

reasonable to expect that greater the s character of an orbital the smaller it is
...

sp 3

sp 2

sp

(ii) Electronegativity of different orbitals
(a) Electronegativity of s-orbital is maximum
...
33
25
s-character in decreasing order and electroneg ativity in decreasing order

Thus sp-hybrid carbon is always electronegative in character and sp 3 - hybrid carbon is electropositive in
character
...


CH 3

CH 2

CH 2

CH

sp 2

sp

Electropositive carbon

Electronegative carbon
having positive charge

(c) Electronegativity of different hybrid and unhybrid orbitals in decreasing order is as follows

s

sp

sp 2

sp 3

p

% s - character in decreasing order and electroneg ativity in decreasing order
...
112Å

sp 3

sp 3 (alkanes)

1
...
103Å

sp 2

sp 2 (alkenes)

1
...
08Å

sp

sp (alkynes)

1
...
(1
...

(iv) Bond strength in hydrocarbons : The shorter the bond, the greater the compression between
atomic nuclei and hence greater the strength of that bond is
...
e
...
e
...
This is evident by the bond energies of the various types of C

H and C

C

bonds
...

(b) Acidity of hydrogen is directly proportional to the electronegativity of atom on which hydrogen is
present
...
33

pKa

25

44

CH 3

CH 3

25
50

s- character in decreasing order and acidity in decreasing order
Note :  Acidity

Ka and Acidity

1
( pKa
pKa

log Ka)

 Order of acidic nature of alkynes is, HC

CH

HC

C CH 3

 The relative acidic character follows the order;

H 2O

ROH

HC

CH

NH 3

CH 2

CH 2

CH 3

CH 3

Obviously, the basic character of their conjugate bases follows the reverse order, i
...
,

CH 3 CH 2 CH 2


CH  NH 2 HC

C  RO  HO 

Dipole moment of Organic Compounds
...
e
...
The degree of polarity of a bond is called dipole moment
...


e l
Where, e

magnitued of separated charge in e
...
u
...
e
...

The dipole moment is denoted by arrow head pointing towards the positive to the negative end (↦)
...
Dipole moment of symmetrical compound is always zero, (

0 )
...
Examples

:

CH

Symmetrica l molecules

(ii) Central atom should have no lone pair of electrons
...


Examples : CCl 4 , CH 4 , BH 3 , CO 2
Symmetrica l molecules


...



...

(3)

electronegativity of central atom or surrounding atoms present on the central atom of the

molecule
...
62 D

= 1
...
03

0

 Alkynes has larger dipole moment because the electronegativity of sp C is more than that of
sp 2

(4)

C
...


(5) Dipole moment of the trans derivative of the compound (a)(b)C

C(a)(b) will only be zero if both a

and b will be in the form of atoms
...


If group have non-linear moments, then the dipole moment of the trans isomer will not be zero
...

H 3C

CH 3

Example :

C

H

CH 3

C

H

C
H

C

H 3C

0
...

If value of

will be more, then cos

o derivative

will be less
...

On account of the presence of bulkier groups at the reaction centre, they cause mechanical interference
and with the result that the attacking reagent finds it difficult to reach the reaction site and thus slows down the
reaction
...

(1) Tertiary alkyl halides having bulky groups form tertiary carbocation readily when hydrolysed because
of the presence of the three bulky groups on the carbon having halogen
...
To release the strain it converts into carbocation
...


(3) Steric strain inhibits the resonance
...

Intermolecular forces
...
These ions are held together by very powerful
electrostatic forces are known as inter ionic forces
...
These molecules are held together by very weak forces are known as
intermolecular forces or secondary forces
...


(2) Van der Waal’s forces
...


(1) Dipole – Dipole Forces : These forces exist between

polar molecules which have permanent

dipoles
...


Magnitude of DF depends on the dipole moment ( ) of the bond of the compound and intermolecular distance
1
(i
...
these forces are effective only over short distance)
d4

(d), DF

CH 3

Example

d in increasing order,

Cl , CH 3

Br , CH 3

I

in decreasing order and DF also in decreasing order

(2) Vander Waal’s forces : These forces exist between non-polar molecules
...
Magnitude of VF depends on the number of electrons (e ) and protons (p) in the
molecule as well as on the intermolecular distance (d),
VF

number of e and p;

1/d7 ;

MW;

Surface area of the molecule and

Symmetry of the molecule (symmetry of molecule decreases intermolecular distance (d))
...

Examples of H-bonding in between the two molecules of same or different compounds are
H – O --- H – O
H
H
R
R
H – F --- H– F
H
R
H
Hydrogen fluoride
(associated)

|

|

H

H

|

|

H – N --- H – N

|

|

|

|

O – H --- O – H

O – H --- O – H
Alcohol and Water

Water

|

|

Alcohol

(associated)

H

H

(associated)

Ammonia

|

O–H
|

H – N --- H
|

a

H

a

Ammonia and water

(associated)

Nature and Importance of Hydrogen bonding
(i) Hydrogen bond is merely an electrostatic force rather than a chemical bond
...

(iii) Bond energy of hydrogen bond is in the range of 3 to 10 kcal/mol or 10 to 40 kJ/mol, i
...
, about 1/10th
the energy of a covalent bond
...

(v) All the three atoms in X – H- - - X lie in a straight line
...


The relative order of these intermolecular forces is,
Hydrogen bonding > dipole-dipole forces > Vander waal’s forces
...

When a chemical reaction takes place between two or more chemical species, new products are formed
...
In a chemical equation, reactants are written on the left
hand side while the products are written on the right hand side
...
The
reactants normally consists of two species,
(1) Substrate : The species, which is attacked by some other chemical species, is called a substrate
...
Thus, Substrate + Reagent

Products
...
In certain cases, a relatively energetically more stable species
than the activated complex may also be formed
...
Thus, a chemical reaction,
in general, may follow either of the following two paths,
Path I : Substrate + Reagent

Activated complex

Products

Path II : Substrate + Reagent

Activated complex

Intermediate

Products
...
If the reaction mechanism involves the breaking and making of bonds simultaneously
without the formation of any intermediate, it is called concerted mechanism
...


Reactants
Products
Progress of reaction

Concerted mechanism (formation of
activated complex only)
...


Enthalpy curves for concerted and Non-concerted mechanisms
To understand clearly the mechanism of various organic reactions, it is essential to have knowledge
about the following concepts;
Electronic displacements in covalent bonds,
Cleavage (fission or breaking) of covalent bonds,
Nature of attacking reagents
...

It is observed that most of the attacking reagents always possess either a positive or a negative charge,
therefore for a reaction to take place on the covalent bond the latter must possess oppositely charged centres
...
The electronic
displacement in turn may be due to certain effects, some of which are permanent and others are temporary
...

Electronic displacement

Polarisation effect (permanent)

Polarisability effect (temporary)

Inductive

Mesomeric

Hyperconjugative

Inductomeric

Electromeric

effect

effect

effect

effect

effect

Inductive effect or Transmission effect

(1) When an electron with drawing (X) or electron-releasing (Y) group is attached to a carbon chain,
polarity is induced on the carbon atom and on the substituent attached to it
...
This is called inductive effect or simply as I –
effect
...

(ii) It operates through

bonds
...

(iv) The shared pair of electrons although permanently shifted towards more electronegative atom, yet
remains in the same valence shell
...


(vi) The inductive effect is not confined to the polarization of one bond but is transmitted along a chain of
carbon atoms through

bonds
...


(vii) Inductive effect brings changes in physical properties such as dipole moment, solubility, etc
...

(viii) Carbon-hydrogen bond is taken as a standard of inductive effect
...
Atoms or groups which have a greater electron withdrawing capacity than hydrogen are said to have–I
effect whereas atoms or groups which have a greater electron releasing power are said to have +I effect
...
alkyl > sec
...
alkyl > CH 3

H

+ I power in decreasing order with respect to the reference H
number of carbon’s in same type of alkyl groups

+ I power

CH 3

CH 2

CH 2

CH 2

CH 3

CH 2

CH 2

CH 3

CH 2

+ I power in decreasing order in same type of alkyl groups
(3) Application of Inductive effect
(i) Magnitude of positive or negative charge : Magnitude of +ve charge on cations and magnitude of
–ve charge on anions can be compared by + I or – I groups present in it
...

CH 3
H 3C

CH 3

C

X

H3C

CH

X

CH 3

CH 2

X

CH 3

X

CH 3

Tertiary

>

Secondary >

Primary

> Methyl

(iii) Dipole moment : As the inductive effect increases, the dipole moment increases
...
64 D

I

CH 3

Br

1
...
83 D

Inductive effect increases
(iv) Relative strength of the acids (Acidic nature of

COOH )

(a) An acid may be defined as a species that has the tendency to loose proton
...

O
||
R C OH ⇌ R

O
||
C O H

(b) The relative strength of acids are measured in their ionisation constants ( K a or pK a values)
...

(c) Any group or atom showing +I effect decreases the acid strength as it increases the negative charge
on the carboxylate ion which holds the hydrogen firmly
...

Thus, acidic nature is, HCOOH

CH 3 COOH

C2 H 5 COOH

C3 H 7 COOH

C4 H 9 COOH

(+ Inductive effect increases, so acid strength decreases)
Formic acid, having no alkyl group, is the most acidic among these acids
...
Greater is the number of such atoms or groups (having – I effect), greater is the acid
strength
...
Thus,
HCOOH

RCOOH

C6 H 5 COOH

Acid strength in decreasing order
Note :  Decreasing order of acids : NO2 CH 2 COOH

F3 C COOH

Cl3 C COOH

Br3 C COOH

FCH2 COOH

ClCH 2 COOH

BrCH 2 COOH
...


(v) Acidity of alcohols : Acidity of alcohol depends on the stability of alkoxide ion (i
...
, conjugate base of
alcohol) which is obtained by the dissociation of alcohols
...

CH 3 OH

CH 3 CH 2 OH

(CH 3 )2 CHOH

(CH 3 )3 COH

Methyl
alcohol

Ethyl
Alcohol

Iso propyl
alcohol

Tert butyl
alcohol

As compared to water, phenol is more acidic (–I effect) but methyl alcohol is less acidic (+I effect)
...
Base strength is defined
as the tendency to donate an electron pair for sharing
...

BOH ⇌ B
Base

OH ; Kb

[B ][OH 
]
; pKb
[BOH ]

log Kb

Greater the value of K b or lower the value of pKb stronger will be the base
...

The +I effect increases the electron density while –I effect decreases
...
“So more is the tendency to donate electron pair for coordination with proton, the more is basic
nature, i
...
, more is the negative charge on nitrogen atom (due to +I effect of alkyl group), the more is basic
nature”
...
The reason is believed to be
steric factor
...
e
...


(d) As compared to ammonia, methylamine is more basic (+I effect) but aniline is less basic and
diphenylamine is still more weaker (– I effect)
...


(CH 3 )3 COH
(3 o )

(CH 3 )2 CHOH
(2o )

CH 3 CH 2 OH
(1o )

CH 3 OH
methyl / alcohol

In Lucas test of making distinction between three types (1o ,2 o and 3 o ) of monohydric alcohols,

R OH HCl(conc
...
ZnCl 2

R Cl

(White ppt
...
HCl in presence of anhydrous ZnCl2 (Lucas reagent) and gives white
turbidity immediately
...


(CH 3 )3 C

(CH 3 )2 CH

CH 3 CH 2

CH 3

(ix) Stability of carbanion : Stability of carbanion increases with increasing – I effect
...
In case the compound in question is having conjugated system
of double bonds, the mesomeric effect is transmitted through whole of the conjugated system and thus the
effect may better be known as conjugative effect
...

(ii) It is operative in unsaturated compounds especially having conjugated systems
...
This effect operates through conjugative

mechanism
...

(v) It affects the physical properties such as dipole moment, solubility etc
...

(vi) Groups which have the capacity to increase the electron density of the rest of the molecule are said
to have

M effect
...
Groups which decrease the electron density

of the rest of the molecule by withdrawing electron pairs are said to have

M effect, e
...
,

(a) The groups which donate electrons to the double bond or to a conjugated system are said to have

M effect or

R effect
...

Cl :

...


CH

Vinyl chloride

In vinyl chloride C

M effect groups :

Cl bond has double bond character due to resonance
...


...


(b) The groups which withdraw electrons form the double bond or from a conjugated system towards
itself due to resonance are said to have

M effect or

R effect
...


O
||
N , C , CHO , COOH , SO 3 H

(vii) The inductive and mesomeric effects, when present together, may act in the same direction or
oppose each other
...
For example, in vinyl chloride due
to – I effect the chlorine atom should develop a negative charge but on account of mesomeric effect it has
positive charge
...

: Cl

...


CH


...

(4) Difference between Resonance and Mesomerism : Although both resonance and mesomerism
represent the same phenomenon, they differ in the following respect : Resonance involves all types of electron
displacements while mesomerism is noticeable only in those cases where a multiple bond is in conjugation
with a multiple bond or electron pair
...

:O
||

(ii) R

C

CH

CH


...

:O:


...


||

R

C

O H

...
Let us consider the following
example H


...




H Cl
...

Hyperconjugative effect
(1) When a H

C bond is attached to an unsaturated system such as double bond or a benzene ring,

the sigma ( ) electrons of the H

C bond interact or enter into conjugation with the unsaturated system
...
The concept of
hyperconjugation was developed by Baker and Nathan and is also known as Baker and Nathan effect
...
Since there is no bond between the
carbon atom and one of the hydrogen atoms, the hyperconjugation is also called no-bond resonance
...
In this, the electron pair of C
involved in conjugation with the

H bond (

bond
...

(3) Structural requirement for hyperconjugation

bond

Hyperconjugation

C

H–C Bond

C

C
Unsaturated system
Orbital representation of - hyperconjugation

-

(i) Compound should have at least one sp 2 -hybrid carbon of either alkene alkyl carbocation or alkyl free
radical
...


If both these conditions are fulfilled then hyperconjugation will take place in the molecule
...


C H3

CH

CH 2

CH 3

CH
|
CH

CH

CH

2

3

(b)

CH 3
|
CH 3 C CH
|
CH 3

CH 2

No
...

C H3

C H3

C H2

CH

C H3

CH 3

C

C H3

|

CH 3

(c)

(C – H), odd electron conjugation : This type of conjugation occurs in alkyl free radicals
...

C H2

C H3

C H3

sp 2


...


H

H
|
C CH
|
H

H
CH 2

H

C
|
H



CH

CH

2

H
|
HC
|
H



CH

CH

2

H

Number of -hydrogens



CH

CH

-hydrogens + 1
...

Stability of alkenes

CH 3

CH

Number of alpha hydrogens

CH 2

CH 3

CH 2

CH

Number of resonating structures
CH 3

CH 2

CH CH
|
CH 3

CH 2

Stability in decreasing order
CH 3

CH 3
C

CH 3

C

CH 3
C

CH 3

CH

CH 3

C

CH 3

CH 3

CH 2

CH 3

Number of alpha hydrogens in decreasing order
stability of alkenes in decreasing order
(ii) Carbon-carbon double bond length in alkenes : As we know that the more is the number of
resonating structures, the more will be single bond character in carbon-carbon double bond
...
Examples,

Number of -

Number of

Carbon-carbon

hydrogens

resonating structures

double bond length
in Å

Zero

CH 3

CH

CH 2

CH 3

CH 2

CH

CH 3

CH
|
CH

CH

CH 2
CH 2

1
...
39 Å

2

3

1
...
35 Å

Zero

CH 2

Zero

1
...


number of resonating structures

C H3

CH 3

C H2

CH 3

CH

CH 3

CH 3

C CH 3
|
CH 3

Number of -hydrogens in increasing order
Stability in increasing order
(iv) Stability of alkyl free radicals : Stability of alkyl free radicals can be explained by hyperconjugation
...


...
of resonating structures


...

CH

4

CH 3

CH 3

7


...


H

H
|
C H
|

H

H
|
C
||

H

H

H
|
C
||

H

Θ

H

H
|
C
||

H

Θ

Θ
The electron donating power of alkyl group will depends on the number of resonating structures, this
depends on the number of hydrogens present on -carbon
...

(vi) Heat of hydrogenation : It is the heat evolved/mol in the addition of hydrogen to form a saturated
hydrocarbon
...


E (kcal)

Lesser the heat of hydrogenation, lesser is the internal energy and more is stability of the system
...
For ethylene it is 32
...
1
kcal/mol respectively
...

The increase in dipole moment, when hydrogen of formaldehyde (
group, i
...
, acetaldehyde (

2
...
72 D) can be referred to hyperconjugation, which leads to development of

charges
...
27 D)

O

H

H
|
C CH
|
H
(

H

O

H

2
...


(5) Reverse Hyperconjugation : The phenomenon of hyperconjugation is also observed in the system
given below,
X
|
C C
|

C ; where X

halogen

In such system the effect operates in the reverse direction
...


Cl

Cl
|
C CH
|
Cl



Cl

CH 2

Cl

C
|
Cl



CH

CH



Cl

2

Cl
|
C
|
Cl

Cl
|

CH

CH

2

Cl

C

CH

CH

2

Cl



The meta directing influence and dectivating effect of CX 3 group for electrophilic aromatic substitution
reaction can be explained by this effect
...

H
H

Example,

H

HO H
H

C

HO
...
Cl

Cl

Cl

HO

C H
H

H

H

In methyl chloride the –I effect of Cl group is further increased temporarily by approach of hydroxyl ion
...
It is denoted as E-effect and represented by a curved
arrow (

) showing the shifting of electron pair
...
However as soon as the
attacking reagent is removed, original electronic condition is restored
...


(iv) The effect involves the complete transference of

electrons
...

(vi) In this there is complete charge separation and ions are formed
...
e
...


effect
...


C

Since,

C

H

C

C
|
H

;

CH 3

CH

CH 2 H

CH 3

CH

Propene

CH 3 group is electron repelling, the electrons are transferred in the direction shown
...


CH 3

(b) When the transfer of electrons takes place away from the attacking reagent, the effect is called

E

effect
...


C

O

CN 

C O
|
CN

The attacking reagent does not attached to that atom on which electrons have been transferred
...

(i) When the groups linked to a multiple bond are similar, the shift can occur to either direction
...

H 2C

H 2C

CH 2


...

H2 C C H2

Both are similar

(ii) When the dissimilar groups are linked on the two ends of the double bond, the shift is decided by the
direction of inductive effect
...

C H2

…
...

CH

C H2

…
...
(a) way and
not by Eq
...

In the case of carbonyl group, the shift is always towards oxygen, i
...
, more electronegative atom
...

C O

In cases where inductive effect and electromeric effect simultaneously operate, usually

electrometric effect predominates
...

Breaking of covalent bond of the compound is known as bond fission
...
This leads to the formation of odd electron species known as free radical
...

A


...

(iii) Homolysis takes place in gaseous phase or in the presence of non polar solvents (CCl4 , CS 2 ) ,
peroxide, UV light, heat ( 500 o C) , electricity and free radical
...

(2) Heterolytic bond fission or heterolysis
(i) In heterolysis, the covalent bond is broken in such a way that one species (i
...
, less electronegative) is
deprived of its own electron, while the other species gains both the electrons
...
In case of organic compounds, if positive
charge is present on the carbon then cation is termed as carbocation
...

(ii) The factor which favours heterolysis is a greater difference of electronegativity between A and B
...

(iv) The energy required for heterolysis is always greater than that for homolysis due to electrostatic
force of attraction between ions
...

Short lived fragments called reaction intermediates result from homolytic and heterolytic bond fission
...

Free radicals
A free radical may be defined as an atom or group of atoms having an odd or unpired electron
...
) against the
symbol of atom or group of atoms
...

Cl ;

Chlorine
free radical


...

H3 C ;
Methyl
free radical

H
|
...

C
|
C6 H 5

Triphenyl methyl
radical

The first stable organic free radical was triphenyl methyl radical and it was prepared by Gomberg
...


(i) The formation of free radical is initiated by heat, light or catalysts
...


...

C H2


...

H

(d)

(primary)

CH 3
CH

94 kcal

H

CH 3


...

H

CH 3
(secondary )

CH 3
(e) CH 3
CH 3

C

H

91 kcal

CH 3
CH 3


...

C H2


...

C H2

88 kcal


...

C H3


...

(iii) There are seven electrons in the out most orbit of carbon of carbon free radicals
...
Due to this
reason free radicals are highly reactive
...

(vi) Chemical reactions which takes place with the formation of free radical as an intermediate are known
as free radical reactions
...

(a) Initiation step : In this step, homolytic bond fission takes place in the presence of initiator, i
...
,
peroxide, hv , heat etc
...

Cl

Cl

hv or


...

Cl

H

ve

(b) Propagation step : Propagation step is always two or more than two step process and all
propagation steps should be exothermic, otherwise free radical reactions would not take place
...
For
example,
CH 3

H


...

C H3

H

HCl

negative

Propagation step II : Reaction intermediate reacts with reagent to give the product
...

C H3

Cl

Cl

CH 3

Cl


...


...

Cl Cl

Cl 2

Reaction velocity of free radical reaction is very-very high
...
Common

inhibitors are O 2 , I2 , p-benzoquinone and diphenylamine
...
e
...


R
|
...

C
|
H

Secondary
free radical

R

H
|
...

C
|
H

Methyl radical

Benzyl and allyl free radicals are resonance stabilized hence are more stable than alkyl free radicals
...



...


CH 2

CH 2

CH 2

...

C H2


...
In general, the
smaller the amount of energy required for bond breaking, the more stable is the radical
...
Thus, free
radicals have a planar structure with odd electron situated in the

p-orbital

↿

Free or unpaired
electron

unused p -orbital at right angles to the plane of hybrid orbitals
...


Sp

2

hybridised c-atom


...

C H2F


...

C F3
...

(4) Stereochemistry of free radicals : To learn about the stereochemistry of free radical reaction
...
The reaction products are,
H

Cl

C

Et
CH2Cl

Me

Me

C

Cl2
hv

CH2Cl

C

+

Me

Et

Et
Cl

CH2Cl

If we review the insight of the reaction, it is clear that the reaction involves racemization at the reaction
centre
...
Obviously a Cl2 molecule could attack either the
upper or lower lobe of the p -orbital with equal chances, leading to racemic mixture
...

Carbonium ions (carbocations)
Carbocation is defined as a group of atoms which contain positively charged carbon having only six
electrons
...
It is denoted by putting
a positive charge (+) against the symbol of group of atoms
...
Butyl carbonium ion

ion
Primary

Secondary

Tertiary

(1) Characteristics of carbocations
(i) The formation of carbocations can be done,


(a) By heterolysis of halogen compounds : (CH 3 )3
(b) By protonation of alkenes or alcohols : CH 2

C
H

Cl

CH 2 ⇌ C H 2

(CH 3 )3 C C l
CH 3

H

R

O

H ⇌ R

(c) By decomposition of Diazo compounds : C 6 H 5

N2

–H2O

O H2 ⇌ R
Cl

Cl

H 2O

C6 H 5 N 2

N2

C6 H 5

N2

(ii) Carbocations are not electrically neutral
...
e
...

(iv) A carbocation is diamagnetic in nature due to all paired electrons
...
e
...
The order of
reactivity of carbocations is, Primary (1 o) > Secondary (2o) > Tertiary (3o)
(2) Types of carbocations : Carbocations can be classified into the following types,
(i) Alkyl carbocations
(a) When positive charge is present on the alkyl carbon, carbocation is known as alkyl carbocation
...

(d) According to these two effects the stability order is as follows :
R CH

R C R
|
R

R C H2

R

C H3

Stability in decreasing order
(e) If

-atom with respect to carbocationic carbon has one or more than one lone pair of electrons then

lone pair of electrons strongly stabilises a carbocation due to the delocalisation
...

CH 3
CH 3


...

O C H 2 is more stable than R C R
|

...

(iii) Allyl carbocation : (CH 2

CH

C H2)

(a) When positive charge is present on the allylic carbon of the allyl group, the carbocation is known as
allyl carbocation
...
Allyl
carbocations are stabilised by delocalisation (or resonance)
...
Thus, tertiary is more stable than secondary which is more stable than primary allyl
carbocation
...

(b) Phenyl methyl carbocations are of three types,
C6 H 5

C H2

C6 H 5

CH

C6 H 5

C6 H 5

C C6 H 5
|

C6 H 5

Benzyl carbocation

Diphenyl methyl

Triphenyl methyl

or

carbocation

carbocation

Phenyl methyl carbocation

(2o-carbocation)

(3o-carbocation)

(1o-carbocation)
(c) Stability of phenyl methyl carbocations can be explained by resonance
...
of resonating
structures :

10

7

(i) Number of resonating structures in decreasing order
(ii) Stability in decreasing order
...

(v) Aromatic carbocations
(a) Cations in which positive charge is present on carbon of aromatic system is known as aromatic
carbocation
...
For example tropolium
carbocation as tropolium bromide is a yellow solid
...

(c) Cations obeying Huckel (4n+2) rule are stable because they are aromatic and there is complete
delocalisation of positive charge
...
They are more stable than benzyl carbocations
...
Thus additional
cyclopropyl group has commulative additive effect on the stability
...

(3) Relative : Stability of different types of carbocations in decreasing order

C

(C6 H 5 )2 C H

(C6 H 5 )3 C

>

C6 H 5

>

CH

C H2

CH 2

CH

>

C H2

Vacant orbital
unhybridised
o

120

R

Note
formed
...
The three sp 2 hybrid
orbitals form three

bonds with hydrogen or other carbon atoms of alkyl groups
...
Since it involves sp 2 hybridisation, its structure is planar with a bond angle of 120 o
...
g
...

(6) Rearrangement in carbocations
(i) Rearrangement takes place in primary and secondary carbocations
...
carbocation

Driving force for rearrangement is the stability factor (Less stable species wants to convert into more
stable species)
...
( ,
(iii) If

CH 3

to

-carbon
...
2 H shift

CH 3

p Carbocatio n

CH 3

)
...
If

and

CH 2

C CH 3 ; CH 3
|
CH 3

CH 3
|
C C H2
|
CH 3

to

-carbon
...
2 CH3 shift

CH 3

C CH 2
|
CH 3

CH 3

ter Carbocatio n

1
...


CH 3

CH 3
|
C CH 2
|
CH 3

Cl

NaOH

CH 3

CH 3
|
C CH 2 OH
|
CH 3

CH 3

OH
|
C CH 2
|
CH 3
Rearranged product

Carbanions

CH 3

A carbanion may be defined as a species containing a carbon atom carrying a negative charge
...

It is denoted by putting a negative charge (–) against the symbol of group of atoms
...
butyl carbanion

Secondary

Tertiary

Primary
(1) Characteristics of carbanions,
(i) The formation of carbanions can be done,

(a) By attack of base on hydrogen atom : Organic compounds which possess a labile or acidic hydrogen
have the tendency to produce carbanions as in the case of reactive methylene compounds which lose proton
in presence of sodium ethoxide (C2 H5 ONa)
...

(iii) There are eight electrons in the outermost orbit of carbanionic carbon hence its octet is complete
...

(v) Carbanions are charged nucleophiles i
...
, they react with electrophilic reagents
...

Stability in inceasing oder
...

Stability in decreasing order
...


For example :


I

C H2



Br

C H2



Cl



C H2

F

C H2

– I power of halo group is in increasing order
Stability is in increasing order
(iii) Delocalisation or Resonance : Allyl and benzyl carbanions are stabilised by delocalisation of
negative charge
...

(iv) Stabilisation by sulphur and phosphorous : Attachment of carbanionic carbon of a sulphur and
phosphorus atom causes an increase in carbanion stability
...


(v) Stabilisation by >C=O, –NO2 and CN groups present on carbanionic carbon :These groups
stabilise carbanion by resonance effect
...

(vi) Stability of Aromatic carbanions
(a) Anions in which negative charge is present on carbon of aromatic system is known as aromatic
carbanions
...

(c) Anions obeying Huckel rule are stable because they are aromatic and there is complete delocalisation
of negative charge
...
Therefore, it has a pyramidal structure
...
The fourth sp 3 hybrid orbital contains the lone pair of
electrons
...

C

sp3 Hybrid
orbital
containing lone
pair

Pyramidal structure of
carbanion

hybridisation is sp 2
...


 The carbanions having three different bulky groups are not optically active
...

R

R

R

:

:

C

C

R

R

R

(5) Reactions involving carbanions
(i) Condensation reactions of carbonyl compounds, i
...
, Aldol condensation, Perkin reaction, Reformatsky
reaction etc
...

(iii) Witting reaction
...
So

Y


...


...
Substituted carbenes are simply named as

derivative of carbenes
...

CH

R


...

C R

Dialkyl carbene

Phenyl carbene

Dichloro carbene

(1) Characteristics of carbnes
(i) Carbenes can be formed,
(a) By the photochemical decomposition or pyrolysis of aliphatic diazo compounds or ketenes
...

CHCl 3
Chloroform

C 2 H 5 ONa

: CCl 2
Dichloro carbene

CHCl 3

(CH 3 )3 CO  ⇌ (CH 3 )3 C

C 2 H 5 OH

OH : CCl 3

NaCl

: CCl 2

Cl

Tert
...


N2

alc
...

CX 2

X

(iii) Carbenes are neutral, divalent, highly reactive carbon intermediates
...

(2) Types and stability of carbenes
Carbenes are of two types
(i) Singlet : When both the electrons go into one orbital and have opposite spin (antiparallel), i
...
, the
unshared electrons are paired in one orbital and has a bent strucutre
...
12Å
...

(ii) Triplet : When the two electrons go into different orbitals and have same spin (parallel), i
...
, the
unshared electrons are not paired
...
Such a state is called triplet state
and is believed to be a linear molecule
...


o

and a C

H

C

H

;

R

C

R

H bond length of 1
...
g
...

R

R
C

R

C

CH 2

H

R

C C

Carbene
(Triplet)

H

R

H
C C

H

H

H

R

CH 2

CH 2

(Cis )

Cis- alkene

(Trans )

(e) In case of conjugated or cumulative dienes, the addition always occurs exclusively at 1,2-position
...


H bond, e
...
,

CH 3 CH 2 CH 2 H

CH 3 CH 2 CH 2 CH 3 and CH 3 CHCH 3
n butane
|
CH 3

: CH 2

Propane

Iso butane

(iii) Ring expansion reactions : These involve the addition of a halogeno carbene across a double
bond followed by rearrangement
...


RNH 2
Mechanism :

CHCl 3

CHCl 3
OH
( H 2O )

3 KOH

RNC

3 KCl

3H 2 O



: C Cl 3

: CCl 2
Dichloro carbene

Cl

(v) Reimer-Tiemann reaction : Phenol on heating with chloroform and aqueous alkali followed by acid
hydrolysis to yield salicylaldehyde is known as Reimer-Tiemann reaction
...

Ph3 C

CH 2
1

o


...

CH 2


...

CH 2

3o

2o

CH

PhCH 2



...

(2) It is neutral reaction intermediate derived from benzene ring by removing two substituents, of ortho
positions, one in the form of electrophile and other in the from of nucleophile leaving behind two electrons to be
destributed between two orbitals
...

(4) When halobenzene is heated with sodamide formation of benzyne takes place
...



(ii) It reacts with strong nucleophile like NH 2


NH2

N H2

NH2

H


...

(2) There is posibility of two spin states for nitrenes depending on whether the two non-bonding electrons
(the normal nitrogen lone pair remains paired) have their spins paired or parallel
...

R –
...

These two may be paired or unpaired

(3) In general nitrenes obey Hunds rule and are ground state triplet with two degenerate sp -orbitals
containing a single electron each
...

C N

...

R C N

...

R


...

N:

or hν


...

N :) can be obtained by photolysis of (or by passing electric discharge

(iii) Unsubstituted nitrene (H
through) NH 3 , N 2 H 4 or N 3 H
...


Most of the attacking reagents carry either a positive or a negative charge
...
The fission of the substrate
molecule to create centres of high or low electron density is influenced by attacking reagents
...

(1) Electrophiles or electrophilic reagents
...

(1) Electrophiles : Electron deficient species or electron acceptor is electrophile
...


H ,X ,R ,N

O
,N

...



...


BeCl 2 , BH 3 , ZnCl2 , AlX 3, FeX 3 , CH 3 , CH 2 , CX 2
(b) Neutral covalent compound in which central atom has complete or expended octet and central atom
has unfilled –d-sub-shell is neutral electrophile,

SnCl 4 , SiCl 4 , PCl5 , SF6 , IF7
(c) Neutral covalent compound in which central atom is bonded only with two or more than two
electronegative atoms is neutral electrophile
...

PCl 5 , NF3 , C X 2 , CO 2 , SO 3 , CS 2 ,

Note : 

Cl2 , Br2 and I2 also behave as neutral electrophiles
...


(2) Nucleophiles : Electron rich species or electron donor is nucleophiles
...







H, O H, R



 



O, C H 3 , X , S H , R

S

(ii) Neutral nucleophiles : It can be classified into two categories :
(a) Neutral covalent compound, in which central atom has complete octet, has at least one lone pair of electrons
and all atoms present on central atom should not be electronegative, is neutral nucleophile
...



...



...


N H 2 (Nitrogen nucleophile)


...



...



...


H

S H, R

S H, R

S R (Sulphur nucleophiles)


...



...



...



...



...


P H 3 , R P H 2 , R 2 P H , R 3 P (Phosphorus nucleophiles)
(b) Organic compound containing carbon, carbon multiple bond/ bonds behaves as nucleophile
...


C

N, O N

O


...


 Nucleophiles are Lewis bases
...


 Strong Lewis acids stronger is electron phile CO 2
conjugated base or weaker is nucleophile
...
Stonger is an acid weaker is its

F

OH

NH 2

CH 2

Increasing order of nucleophilicity
...

It is convenient to classify the numerous reactions of the various classes of organic compound into four types,
Substitution reactions,

Addition reaction,

Elimination reactions,

Rearranged reactions,

Substitution reactions
Replacement of an atom or group of the substrate by any other atom or group is known as substitution reactions
...
)

HNO 3

Benzene

H

(Conc
...


R

X

Substrate

OH 

Nucleophil e

R

OH

X

Leaving group

Such substitution reactions are called nucleophilic substitution reactions, i
...
, S N reactions (S stands for
substitution and N for nucleophile)
...


1
Basicity of the group

Leaving power of the group

 

HI

Example :

HBr

HCl

 

I Br Cl F

HF

Increasing basicity

Decreasing acidity

Decreasing leaving ability
(iii) The leaving power of some nucleophilic groups are given below in decreasing order,
O
||

CF3

O



S O
||

Br

CH 3

||

O

Br

||

CF3

||

||

C6 H 5



H3 O

O



Cl

F

||

CH 3

O



S O
||

O



C O

O



S O

O



I

||

S O

O


O



||



||

CH 3

S O
||

O

O



C O

(iv) In these reactions leaving group of the substrate is replaced by another nucleophile
...
Negative part of the reagent is always
nucleophilic in character
...
Thus
strongly basic nucleophilic group replaces weakly basic nucleophilic group of the substrate
...

(vi) Unlike aliphatic compounds having nucleophilic group as leaving group, aromatic compounds having
same group bonded directly with aromatic ring do not undergo nucleophilic substitution reaction under
ordinary conditions
...

Another factor for the low reactivity is nucleophilic character of aromatic ring
...

Distinction between SN2 and SN1 reactions
2

Factors
Number of steps

1

SN Reactions
One:

R : L : Nu

R : Nu : L

SN Reactions
Two: (i) R : L
(ii) R

Reaction rate and order

Second order:
Rate
[Substrate] [Nucleophile] or Rate =
Bimolecular

: Nu

R
Fast

: L

R : Nu

First order:
Rate [Substrate] or Rate = K1[RL]

K 2 [RL][: Nu 
]

Molecularity

Slow

Unimolecular

TS of slow step
: Nu

C

: Nu

:L

C

L

Nu :

Reacting nucleophile

The nucleophile attacks the carbon of the
substrate exclusively from the back side
...


Stereochemistry

Complete inversion of configuration takes
place
...


Reactivity order of alkyl halides

Methyl>1°>2°>3°halides
...
(I

Rearrangement

No rearrange product is form (except for
allylic)
...


Nature of nucleophiles

Favoured by strong and high concentration of
nucleophiles
...


Polarity

Favoured by solvents of low polarity
...


Reaction rate determining factor

By steric hindrance
...


Br

Cl

F)

Br

Cl

F)

By electronic factor (stability of R )
...
g
...


(2) Electrophilic substitutions reactions : Electrophilic substitution involves the attack by an electrophile
...
If the order of reaction is 1, it is written as
S E1 (unimolecular)and if the order is 2, it is S E 2 (Bimolecular)
...
g
...

In all electorphilic aromatic substitution reactions, it
involves:

CH 3

CH 3

CH 3

CH 3

Step 1
...
e
...
The electrophile attacks the aromatic ring to form
+
Benzene
carbonium ion (or arenium ion) which is stabilized by
resonance
...
Carbonium ion loses the proton to form substitution
product
...

Similarly, Nitration, sulphonation and Friedel-Crafts
reaction…
...
, in benzene nucleus are the other examples of
electrophilic substitution
...
These reactions occurs by free radical mechanism which involves : Initiation, Propagation and
Termination steps
...

CH 4

Cl 2

Methane

UV

CH 3 Cl
Methyl chloride

light

HCl

The reaction does not stop with the formation of methyl chloride (CH 3 Cl) but the remaining hydrogen atoms are
replaced one by one with chlorine atoms to give rise chain reaction
...


C6 H 5

H

Alkali

C6 H 5 N 2 X

C6 H 5

Benzene diazonium halide

C6 H 5

N2

HX

Diphenyl

(iii) Wurtz reaction : Ethyl bromide on treatment with metallic sodium forms butane, ethane and ethylene by
involving free radical mechanism
...



...


C2 H 5

C2 H 5

2CH 3

C H2


...
This type of reaction involving

substitution at the alpha carbon atom with respect to the double bond is termed Allylic substitution
...
Some examples are:
CH 2

(a) CH 3

CH
Propene

CO

CH 2

CH 2
N

CH 2

CO

Br

CCl4

Br

CH 2

CH

NH

CH 2

Allylbromide

CH 2

NBS

Note : 

CO
CO

Succinimid e

Halogenation of alkanes occurs by a free radical mechanism and is catalysed by radical initiators like

(C6 H 5 COO )2 , Pb(CH 3 )4 , Pb(C2 H 5 )4 etc
...
e
...
In this reaction there is loss of one bond and gain of two bonds
...
The reaction is a spontaneous reaction
...

(1) Electrophilic additions (2) Nucleophilic additions and
(3) Free radical additions
(1) Electrophilic addition reactions
(i) This reaction is mainly given by alkenes and alkynes
...

(iii) Alkenes and alkynes give electrophilic addition with those reagents which on dissociation gives electrophile as
well as nucleophile
...

(v) Unsymmetrical alkenes and alkynes give addition reactions with unsymmetrical reagents according to
Markownikoff’s rule
...

This rule can be used only in those alkenes which fulfil the following conditions:
(a) Alkene should be unsymmetrical
...

(c) If phenyl group is present on doublly bonded carbon, then both doublly bonded carbons should be substituted
by phenyl groups
...

CH 3
CH 3
C6 H 5
C CH 2 ,
C CH CH 3
C CH C 6 H 5 , C6 H 5 CH CH 2
CH 3 CH CH 2 ,
CH 3
CH 3
C6 H 5
Following alkenes will not give addition reaction according to Markownikoff’s rule
...


CH 2

CH

G
O
||

where G is a strong –I group such as

CX 3 , NO 2,

CN , CHO , COR , COOH , C Z (Z

Cl, OH , OR , NH 2 )

Cl
Example: CH 2

CH

CHO

Anti-Markovniko v addition

HCl

|

CH 2 CH 2

(vii) Mechanism of electrophilic addition reactions is as followes,
|

C C E
|

C

Olefin

E

Slow

Electrophi le

|

C C E

|
Carbonium ion

|

Fast

X

C

CHO

C C E

Nucleophil e

|

|

X

Addition product

(2) Nucleophilic addition reactions : When the addition reaction occurs on account of the initial attack of
nucleophile, the reaction is said to be a nucleophilic addition reaction
...
This makes carbon atom of the carbonyl group electron
deficient
...

CH 3

CH 3
C

O HCN

CH 3

OH
C

CH 3

Acetone

CN

Acetone cyanohydri n

The mechanism of the reaction involves the following steps:


Step 1
...


HCN

CN 

H

Step 2
...




CH 3

CH 3

CN

C
CH

3

O

CN
CH 3



C

CH 3

O or

O
C

CH 3

CN

Step 3
...


does not attack the

CH 3
CH 3

|



CN
CH 3

C

O H

NC

CH 3

C
|

OH

or

OH

C
CH 3

CN

CH 3

In C

O compounds, the addition of liquid HCN gives cyanohydrin and the addendum is CN

ion (addition is

catalysed by bases or salts of weak acids and retarded by acids or unaffected by neutral compounds) and not HCN
directly
...
R
...


C 6 H 5 CH 2 COCH 3

CHO

CH 3 COCH 3

COCH 3

COCl

O
||
C CH 3

C6 H 5

COOCH 3

C6 H 5

CONH 2

O
||
C C6 H 5

COOH

(3) Free radical addition reactions : Those reactions which involve the initial attack by a free radical are
known as free radical reactions
...
Free radical reactions generally take place in non-polar solvents such
as CCl 4 , high temperature, in presence of light or a free radical producing substance like O 2 and peroxides
...

Initiation (Formation of free radical)

...

(a) RO OR
2 R O (b) HBr RO
Peroxide

ROH


...


Propagation
...


Br

CH 3

CH

CH 2

Br or

Secondary (2 ) free radical (more stable)


...


C H2

Primary (1 ) free radical (less stable)


...


...


nCH 2

CH 2

Catalyst

( CH 2

Ethylene

CH 2 )n

Polyethyle ne

Elimination reactions
Elimination reactions are formally the reverse of addition reactions and consist in removing the two groups
(Generally, one being a proton) from one or two carbon atoms of a molecule to form an unsaturated linkage or centre
...
e
...

Elimination reactions are classified into two general types,
(I) - elimination reactions or 1, 1-elimination reactions
...

(I)

- elimination reactions or 1, 1-elimination reactions : A reaction in which both the groups or atoms are

removed from the same carbon of the molecule is called - elimination reaction
...


...
KOH/

CHX 3

Alc, KOH/

CH 2 X 2
CH 3 CHI 2

Zn



X H


...

CH 3 CH

ZnI 2

Product of the reaction is halocarbenes or dihalocarbenes
...

(II) - elimination reactions or 1, 2-elimination reactions : Consider the following reactions,


CH 3

CH 2 CH 2 L

CH 3

CH

CH 2

H L

A reaction in which functional group (i
...
, leaving group) is removed from - carbon and other group (Generally
hydrogen atom) from the - carbon is called - elimination reaction
...
Product of the reaction is generally less stable than the reactant
...

Rate [Substrate]
(b) Product formation takes place by formation of carbocation as reaction intermediate (RI)
...

(d) Reaction is carried out in the presence of polar protic solvent
...
CH 3

|

C Cl

CH 3

|



Slow step

C - - - - - Cl

CH 3

|

CH 3

C CH 3 Cl
|

CH 3

CH 3

(TS 1 )

Step 2
...

B H

CH 2

C
|

CH 3

B - - - - H - - - -CH 2 - - - - C

fast

CH 3

|

BH

CH 2

CH 3

CH 3

CH 3

C

TS 2

(f) Energy profile diagram for E1 reaction is,
TS1
E

TS2
RI

Product
Reactant
Reaction coordinate

(ii) E2 (Elimination bimolecular) reaction : Consider the following reaction,

CH 3

CH 2

CH 2

Br

Base ( B )



CH 3

CH

CH 2

H Br

(a) Reaction velocity depends only on the concentration of the substrate and the base used; thus reaction
is bimolecular reaction
...

(c) Rearrangement is not take place in E2 reaction but in case of allylic compound rearrangement is possible
...

(e) The E1 reaction occurs in one step,

...

O
||

Thus - carbon should contain strong –I group, e
...
,

C , NO 2,

N or other carbanion stabilising group
...

(c) - hydrogen should be highly acidic so that it can easily be removed as proton to give carbanion
...
CF3

C2 H5 O

CHCl 2

Fast step



Step 2
...
Major product of the reaction can be known by two emperical rules
...
e
...
Product of the reaction in
this case is known as Saytzeff product
...
KOH /

CH 3

|

HCl

1

CH 3

CH 3

C

CH CH 3

|

CH 3
Saytzeff p roduct

(ii) Hofmann rule : According to this rule, major product is always least substituted alkene i
...
, major product is
formed from - carbon which has maximum number of hydrogen
...

CH 3
CH 3
Br
|

CH 3

|

C CH 2
|

CH 3

2

CH CH 3
1

Alc
...

 In E1cb reactions, product formation always takes place by Hofmann rule
...
In almost all E2

reactions product formation take place by Saytzeff rule
...
g
...
KOH

CH 2 X

H 2C

( HX )

CH 2 ; CH 3

Alc
...
KOH

CH CH 3
|

CH 3

( HX )

CH

CH

CH 3 CH 3

2 - Butene (Major)

CH 2 CH
1 - Butene (Minor)

CH 2

X
(ii) Dehydration of alcohol is another example of elimination reaction
...
H 2 SO 4 or H 3 PO4
are used as dehydrating agents, the mechanism is E1
...

Dehydration is removal of H 2 O from alcohols,

CH 3

CH 2

OH

CH 3

e
...
,

CH 2

CH 2

Conc
...
H 2 SO 4 , 170 C

OH

CH 2
CH 3

( H 2 O)

Note : Dehydration of alcohols is in the order: Tertiary

CH

CH 2

Secondary

Primary

(2 )

(1 )

(3 )

 2° and 3° alcohol by E1 process and 1° alcohol by E2 process
...
CH 2

CH

CH CH 3
|

OH
is easily dehydrated than CH 3

CH 2

CH CH 3 and so
|

OH
OH
>

(iii) Dehalogenation : It is removal of halogens, e
...
,

OH

OH

>

CH 2 CH 2
|

Br

Zn dust

in CH 3 OH , heat
(-ZnBr2 )

|

H 2C

CH 2

Ethylene

Br

Ethylene bromide

(iv) Dehydrogenation : It is removal of hydrogen, e
...
, CH 3

CH CH 3
|

OH

Isopropyl alcohol

Cu , 300 C
( H2 )

CH 3

C CH 3
||

O

Acetone

Rearrangement reactions
The reactions, which involve the migration of an atom or group from one site to another within the molecule
(nothing is added from outside and nothing is eliminated) resulting in a new molecular structure, are known as
rearrangement reactions
...


It is convenient to divide rearrangement reactions into following types:
(1) Rearrangement or migration to electron deficient atoms (Nucleophilic rearrangement) : Those
rearrangement reactions in which migrating group is nucleophilic and thus migrates to electron deficient centre which
may be carbon, nitrogen and oxygen
...

Examples:
(i) Pinacol – pinacolone rearrangement
CH 3

|

CH 3

O CH 3

|

C — C — CH 3

CH 3

|

|

OH

||

|

C C — CH 3

CH 3

OH

30 % H 2 SO 4

|

CH 3
Pinacolone

Pinacol

(ii) Wagner – Meerwein rearrangement

CH 3

CH 3

|

CH 3

C CH 2 OH

CH 3

|

H 2 SO 4

CH 3

|

C

|

CH

CH 3 CH 2

2 Methyl- butene - 2

CH 3

C CH 2 CH 3

2 Methyl- butene -1

Neopentyl alcohol

(iii) Benzilic acid rearrangement

O O
||

C6 H 5

||

NaOH / KOH

C6 H 5 C C C6 H 5

OH
C

Benzil

C6 H 5



COO

Bezilic acid anion

(iv) Wolf rearrangement :

CH 3

COOH

Acetic acid

CH 2 N 2
diazo methan

(v) Allylic rearrangement

CH 3

CH 3

|

CH 3

C
|

CH

CH 2

H

OH
3 - Methyl- but -1- ene - 3 - ol

(vi) Sommelet – Hauser rearrangement

|

CH 3

C

CH CH 2 OH

3 - Methyl- but - 2- ene -1- ol

CH 3

CH 2 COOH
Propinoic acid

CH2NM
e3X

C
H

NaNH 2
Benzyl quaternary ammonium salt

(vii) Hofmann rearrangement : RCONH 2

O-Substituted benzyl tertiary amine

Br2

(viii) Curtius rearrangement : RCON

(x)Baeyer Villiger reaction : CH 3

4 KOH

RNCO

3
Acid azide

H 2 SO 4

HN 3

CO

CH 3

CH 3

RNH 2

2 KBr

K2CO3

2 H 2O

N2

Isocyanate

(ix) Schimdt rearrangement : RCOOH

CH2Nm
e2

3

R

NH 2

COOOH

CO 2

H 2 SO 4

N2

CH 3

COO

CH 3

H 3 CCOOH

(2) Rearrangement or migration to electron rich atoms (Electrophilic rearrangement) : Those
rearrangement reactions in which migrating group is electrophile and thus migrates to electron rich centre
...
Free radical rearrangements are
comparatively rare
...
Aromatic compounds of the type (I) undergo rearrangements in the manner mentioned below,
X–H

X–H

X–Y

Y
+
+

(I)
Y
The element X from which group Y migrater may be nitrogen or oxygen
...
CH2–CH= CH2

N

N

CH2–CH= CH2

Cl
+

dil
...
AlCl3

OH

OH
COCH3
+

COCH3
Phenylacetate (Ester)

OH

o- and p- hydoxy acetophenone



---